Some recent liquid crystal projectors use an LED as a light source for illuminating a liquid crystal panel.
In general, in a liquid crystal projector, a liquid crystal panel needs to be irradiated with polarized light. Output light from the LED is non-polarized light. Thus, if the LED is used as a light source for illuminating the liquid crystal panel, all light rays from the LED need to be polarized in one direction.
FIG. 1 shows a light source unit capable of polarizing all the light rays from the LED in one direction.
As shown in FIG. 1, the light source unit includes LED 100 and polarizer 101 positioned opposite the emission surface of LED 100.
Polarizer 101 may include periodic recessed and protruding patterns which are one-dimensional in a plane or may be formed from a polymer. Polarizer 101 used herein includes recessed and protruding patterns.
Polarizer 101 includes a transmission axis, and transmits light with a polarization component parallel to the transmission axis, while reflecting light with a polarization component orthogonal to the transmission axis. In polarizer 101 with recessed and protruding patterns, the transmission axis generally coincides with the direction of periodicity of the recessed and protruding patterns. In this case, for light incident on polarizer 101, the polarization component parallel to the direction of periodicity of the recessed and protruding patterns is referred to as TM waves (or TM polarized light). The polarization component parallel to a longitudinal direction of the recessed and protruding patterns (which is orthogonal to the direction of periodicity) is referred to as TE waves (or TE polarized light). That is, the TM wave included in light that exits from the emission surface of LED 100 passes through polarizer 101, while the TE wave also included therein is reflected toward LED 100 by polarizer 101.
The emission surface of LED 100 specularly reflects the TE wave from polarizer 101. The reflected light travels toward polarizer 101. With regard to the specular reflection of the TE wave by the emission surface, the incident light and the reflected light have the same direction of polarization. That is, the direction of polarization present before the specular reflection is maintained even after the specular reflection.
According to the above-described light source unit, only the TM wave included in the light exiting through the emission surface of LED 100 passes through polarizer 101. This allows all the light rays output from the light source unit to be polarized in one direction.
However, as shown in FIG. 2, the TE wave, included in the light exiting through the emission surface of LED 100, propagates between the emission surface of LED 100 and polarizer 101 while being repeatedly reflected. The TE wave cannot be used as output light from the light source unit.
When means for polarization-converting the TE wave into the TM wave, for example, a quarter wave plate, is provided between the LED and the polarizer, the intensity of outgoing light from the light source unit can be increased. FIG. 3 shows a light source unit with a quarter wave plate.
As shown in FIG. 3, quarter wave plate 102 is provided between LED 100 and polarizer 101. Quarter wave plate 102 is arranged in proximity to the emission surface of LED 100, which is a heat source. Quarter wave plate 102 thus needs to be formed from a material having high heat resistance. At present, the quarter wave plate 102 is formed from a crystal such as quartz or mica, which offers high heat resistance. Such a quarter wave plate has a thickness set to several hundred μm with processing accuracy or the like taken into account.
As shown in FIG. 4, the TE wave reflected by polarizer 101 is converted into circularly polarized light by quarter wave plate 102. The circularly polarized light then enters the emission surface of LED 100.
The circularly polarized light having entered the emission surface of LED 100 is specularly reflected. The specularly reflected light enters quarter wave plate 102 with the circular polarization maintained. The circularly polarized light from the emission surface of LED 100 is converted into a TM wave by quarter wave plate 102. The TM wave from quarter wave plate 102 passes through polarizer 101.
As described above, the TE wave reflected by polarizer 101 is converted into a TM wave by passing through quarter wave plate 102 twice, that is, when traveling from polarizer 101 toward LED 100 and when traveling from the emission surface of LED 100, by which the TE wave is specularly reflected, toward polarizer 101. Polarization-converting the TE wave into the TM wave in this manner enables an increase in the intensity of outgoing light from the light source unit.
Patent Literature 1 discloses a technique relating to a light source unit including an LED and a polarizer. Patent Literature 2 discloses a technique relating to a light source unit with a quarter wave plate provided between an LED and a polarizer.